1. Field of the Invention
This invention relates to the in situ or underground gasification of coal by drilling boreholes into the major fracture system of the coal, igniting the coal and recovering the gas formed.
2. Description of the Prior Art
In 1971, more than 75 percent of the energy used in the United States came from petroleum and natural gas. Coal, which is our most abundant energy resource, supplied only 18 percent of the total energy consumed in that year. The increasing short supply of petroleum and natural gas has caused many people in the energy field to consider various methods of converting our vast coal reserves to an environmentally acceptable fuel.
Coal gasification has become the subject of many studies in recent years as a conversion technique which shows promise of providing a substitute for high Btu natural gas. Such a substitute gas would have its greatest impact on the industrial sector, primarily in the electric power generation field. Low Btu gas can be produced from coal either above ground or in place.
Since 1969, with the passage of the Federal Coal Mine Health and Safety Act, the Bureau of Mines has increased its emphasis on reducing the occupational hazards associated with coal mining. The in situ gasification of coal has therefore become a possible means of extracting energy value from coal while minimizing health and safety problems.
The production of coal energy by the use of wells through underground mining has been a continual subject of interest in the field of energy production for many years. Coal gasification by use of above ground retorting is well known in the art and was developed in Germany prior to World War II. In this method, oxygen and steam are simultaneously injected into a field retort and upon combustion, a gas having a calorific value sufficient for commercial usage and coal tar liquids is produced.
Various ideas have been advanced for conducting the gasification of coal underground or in situ, that is, to convert the coal "in place" to a usable gas having a high calorific value. Many technological advances were made in this area and efforts were primarily confined to the advancement of theories on the subject until substantial work and testing was done in Russia. Much of the Russian work involved considerable underground mining and construction in an effort to provide the necessary passageways for air to pass through the coal. Some efforts involved breaking up the coal underground to provide air passages but the problem with the Russian approach was that the numerous wells required to break up the coal underground and provide adequate air passages made the approach uneconomical as the amount of excavation encountered was tremendous.
The art then progressed to drilling holes in the coal seam and charging with dynamite. As the burning front progressed through the stratum the charges were automatically set off in an effort to break off and crush the coal and render a segment of the bed more permeable. This resulted in irregularities too great to sustain continued gas flow and the gas produced contained such large amounts of air that the heating value of the gas was lowered substantially.
A further approach involved shaft and borehole mining combinations but these progressed only to the point that steeply sloping seams near outcrops could be utilized to provide a useful gas. A major problem with this system as well as the previous systems discussed involved the large amount of excavation and mining required as well as the further problem that the rate of air injection which directly affected the gas produced was responsible for providing a gas of low Btu value. The low permeability of the coal contributed to this problem.
A good deal of prior art has been written on this subject matter and various aspects of in situ gasification of coal are disclosed in recently issued U.S. patents. Patents which are concerned with this subject matter include U.S. Pat. Nos. 3,563,606; 3,513,913; 3,628,929; 3,709,295 and 3,775,073. In addition, a publication of the Bureau of Mines of the United States Department of Interior, Information Circular 8193, issued 1963, contains a bibliography of the underground gasification of coal for the period 1945-1960.
The present invention is considered to provide a number of advantages in new methods for the in situ gasification of coal and to provide a major advance in this art.